1. Field of the Invention
The invention relates to a sealing method and apparatus for sealing an annulus between an outer surface of an oil and/or gas well tubing or casing hanger and an inner surface of a wellhead housing.
2. Description of the Prior Art
Oil and/or gas wells typically include one or more pipe strings extending downwardly into the earth from its surface. The strings are included one within the other and serve various purposes, such as maintaining the structural integrity of the well and for controlling fluid flow and fluid pressures within the well. A "string" is referred to as casing if there is at least one string within that string, and the innermost string is referred to as tubing.
At the wellhead, various types of wellhead members are connected and sealed to the casing and tubing and perform various functions, among which are: to support the casing and tubing from the surface; to provide means for connecting fluid conduits to the tubing as well as to the annuli defined by the tubing and the various casing strings surrounding it; and for maintaining control of the fluid pressures experienced within the wellhead. To maintain control of the often very high fluid pressures, it is necessary to provide seals between the various wellhead members and the tubing and casing. Elastomeric seals have been provided in such devices which provide a seal against the tubing and casing when the seal is pressed inwardly thereagainst. This is achieved in various devices by exerting pressure vertically against the seal causing it to expand inwardly against the tubing or casing thus to seal it off. The elastomeric seal may be urged inwardly also by pressure exerted upon its outer circumferential surface. For example, such seals have been in use for many years wherein fluid pressure is exerted in an annulus surrounding the outer diameter of the elastomeric seal thus to urge it inwardly. The annulus is connected to the exterior of the device by means of a check valve through which fluid under pressure is introduced. In some types of sealing methods, a liquid plastic under pressure is injected through the check valve for forming the seals, and thereafter the plastic hardens so that the seal is permanently maintained.
Many well operators believe that elastomeric seals may be unreliable under extremes of temperature and high pressures, which may cause them to breakdown, leading to an undesirable failure of the seal. Accordingly, metal-to-metal type seals have been adopted for use in order to overcome the foregoing temperature sensitivity problems of elastomeric materials. Various types of metal-to-metal seals have been proposed; however, they suffer from many disadvantages. Examples of such disadvantages include high setting force loads are required in order to force the seal into engagement. In many instances, complex hydraulically-operated systems are required to energize the seal, and typically include additional tools to lock the seal in the desired sealing engagement. Many metal-to-metal type seals utilize a solid wedge to force the seal into engagement with the desired surface; however, temperatures changes experienced by the wellhead and casing or tubing strings, and differential expansion and/or contraction of the various metal parts associated with such temperature changes, can cause the desired sealing to be lost. Some prior art seals permit movement of some components with respect to each other after achieving the desired sealing, whereby it is possible that undesired movement of some components can cause the seal to fail.
Some types of metal-to-metal seals seal against a tapered surface disposed on the wellhead housing and/or tubing or casing hanger. If there is longitudinal movement of either the seal or the adjacent tapered surface, which can be caused by differential expansion or contraction caused by temperature changes or changing tensile force loads on the casing or tubing, the seal may move off the tapered surface and thus destroy the desired sealing. Another disadvantage associated with many types of metal-to-metal seals is that the seals may not be subjected to an external pressure test, nor can the well operator visually determine if the desired sealing has been accomplished. Another disadvantage is that some types of metal-to-metal seals do not provide for a stored energy preload force which takes advantage of the resilience and the elastic/plastic properties of the metal used to make the seal, so as to constantly urge the seal into the desired sealing engagement with its adjacent surfaces. A further disadvantage with many metal-to-metal seals, which utilize multiple seals, is that the multiple seals are set at the same time, rather than independently of one another. It may be difficult to determine whether or not all of the multiple seals have been properly set into sealing engagement.
Accordingly, prior to the development of the present invention, there has been no single sealing method and apparatus for sealing an annulus between an outer surface of an oil and/or gas well tubing or casing hanger and an inner surface of wellhead housing which: does not require high setting force loads; does not require a complex hydraulically operated system and additional tools to lock the seal in its desired sealing engagement; does not use a solid wedge subject to differential temperature changes which could cause the seal to fail; is not sealed against a tapered sealing surface; permits an external pressure test and enables the well operator to visually determine if sealing has been accomplished; has a stored energy preload force to maintain the desired sealing; sets multiple seals independently of each other; and upon sealing becomes a relatively solid seal assembly, the various components of which are not subject to movement relative to each other. Therefore, the art has sought a sealing method and apparatus for sealing an annulus between an outer surface of an oil and/or gas well tubing or casing hanger and an inner surface of a wellhead housing which: does not require high setting force loads; does not require a complex hydraulically operated system to set the seals, and additional tools to lock the seal in the desired sealing engagement with adjacent surfaces; does not utilize a solid wedge subject to differential temperature changes which can cause the seal to fail; seals upon a straight, non-tapered sealing surface; can be tested by an external pressure test and can permit the well operator to visually determine the setting of the seal; has a stored energy preload force to constantly urge the seal into sealing engagement; sets multiple seals independent of each other; and upon sealing becomes a relatively solid assembly with its individual components not subject to movement relative to each other so as to maintain the desired sealing.